Known telephone hook switch (also known as "switch-hook") mechanism commonly comprise a pair of interlinked arms. When the mechanism is installed in the body of a telephone instrument, an end of each of these arms extends from a respective opening in a handset cradle, for detecting the presence or absence of a handset (ie for detecting whenever the instrument is in an on-hook state or an off-hook state respectively). The arms are pivotally mounted and biased towards extended positions from the handset cradle such that the arms assume these positions in the absence of the handset (the off-hook state). In the presence of the handset (the on-hook state) the arms are depressed by the handset, assuming retracted positions with respect to the handset cradle. The arms are arranged to actuate a hook switch (usually a microswitch) for controlling circuitry of the instrument.
Such known hook switch mechanisms are generally satisfactory but can be prone to inadvertent operation by the depression of just one of the extended arms in the off-hook state. Since the arms are interlinked, such an accidental displacement causes actuation of the hook switch, interrupting or terminating a telephone call in progress. This problem is of particular concern when such mechanisms are used in so-called "one-piece" telephone instruments (instruments in which the handset and body are combined). In the case of one-piece instruments, the arms of the mechanism commonly extend from a lower surface of the instrument such that they are depressed when the instrument is placed on a table, for example (ie the on-hook state). In use (the off-hook state) there is a great danger of inadvertent depression of one or other of the extended arms because the entire instrument is hand-held.